Upgraded Detectors May Soon “See” Colliding Black Holes

In his 1994 book Black Holes and Time Warps, physicist Kip Thorne wrote of the tantalizing discoveries to come in the 21st century. In particular, the existence of gravitational waves—ripples in the fabric of space and time—might soon graduate from theoretical prediction to known fact. And those waves could carry all-important hints about their origins in the motion or collision of extremely massive objects.

“Gravitational-wave detectors will soon bring us observational maps of black holes, and the symphonic sounds of black holes colliding—symphonies filled with rich, new information about how warped spacetime behaves when wildly vibrating,” Thorne wrote.

That time is nearly upon us, he now believes. The California Institute of Technology theorist writes in the August 3 issue of Science that in five years' time, ongoing upgrades to the world's leading gravitational-wave observatories will make those instruments sensitive enough to detect the waves, which would provide yet another confirmation of Einstein's general theory of relativity. The detection would also open up a new regime for studying black holes, those cosmic gluttons whose gravitational pull is so strong that it forms a one-way funnel into their maw.

As of now, astrophysicists can only infer the presence of a black hole by monitoring the environs around the putative object. In the case of Sagittarius A*, in the center of our own Milky Way galaxy, for instance, astronomers can see flares of radiation emanating from the black hole's location, caused by infalling material heating up and radiating outside the event horizon. Stars at the galactic center betray the presence of Sagittarius A* as well—their orbits point to the existence of a nearby compact object with the mass of four million suns.

The strong gravitational-wave signature expected from merging black holes would carry a wealth of information both about the objects involved and about their cataclysmic interaction.

Two major gravitational-wave detector projects have been on the lookout for these spacetime ripples, but so far the search has not produced any results. Both the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo observatory are L-shaped instruments with extremely long arms—four kilometers for the two LIGO facilities in Washington and Louisiana and three kilometers for Italy's Virgo. They rely on long-baseline interferometry, firing lasers down the perpendicular arms to see if one direction has been stretched or compressed relative to the other by a passing gravitational wave. “The advanced LIGO and advanced Virgo interferometers are now being installed and by 2017 should reach sensitivities at which black-hole mergers are observed,” Thorne writes. Sounds like the race is on to detect gravitational waves, one of the biggest prizes in physics.